The present invention relates generally to analog to digital converters and, more particularly, relates to a system for improving the accuracy and resolution of an analog to digital converter.
Digital processing of analog signals becomes an increasingly attractive alternative to analog processing as digital hardware becomes increasingly faster, more sophisticated, and more integrated. Also, digital systems are, in general, inherently more flexible and less sensitive to time and temperature than analog systems and can simulate almost ideal linear systems. As a result, a great effort has gone into the development of analog to digital converters (ADC's) and digital to analog converters (DAC's) for transforming an analog signal to a digital representation of that signal, or vice versa, with ever-increasing accuracy, speed, and resolution.
In the process of analog to digital conversion an analog input signal is allowed to vary continuously over a finite amplitude range and is mapped to a finite number of discrete amplitudes (amplitude quantization) and then sampled at a uniform sampling rate. The input signal dynamic range of the ADC is divided into a specified number of possible discrete amplitudes (quantization levels), where the number of discrete amplitude levels specifies the resolution of the ADC. An ADC having 2.sup.m quantization levels generates an m-bit digital output signal where the value of m defines the resolution of the ADC.
An important characteristic of an ADC is its linearity (or accuracy), which is a measure of the variance of the ADC transfer characteristic, i.e., the characteristic mapping the input signal to the corresponding output signal, from a straight line.
In many measurement applications conversion of a slowly varying signal is required. For these applications, high accuracy and resolution are the most important system specifications while high speed is less critical.
Most existing systems for increasing the accuracy and resolution of an ADC stress analog domain methods to minimize the error due to the presence of internal and external noise sources, and to maximize the accuracy and the time and temperature stability of the analog components in the ADC. Other systems that have used digital processing after conversion have required complicated analog preprocessing.
It is also a known technique to add a small scale dither signal to the ADC input signal to increase the resolution of the ADC. However, a small scale dither does not increase the linearity of the ADC.